Many new types of distributed generation and energy storage products are currently being developed. These devices include, but are not limited to: fuel cells, flywheels, advanced batteries, micro-turbines, Stirling engines, wind turbines, solar cells and double-layer capacitors. Each one of these devices requires a power electronic inverter at its output to make useable AC power, typically 50 or 60 Hz single or three-phase power.
The simplest model of parallel power converters on an AC grid consists of two single-phase AC voltage sources 10, 12 operating at about the same frequency connected to each other through an inductance 14, 16 as shown in FIG. 1. If both AC sources are at the same frequency, phase, and amplitude, there will be no current flowing between the sources, as well as no power flow. A slight phase shift in either source will cause real power to flow from the leading source to the lagging source. If the amplitude is changed for either source, reactive power will then flow from the higher amplitude source to the lower amplitude source. Now, if we add a resistive load 18 between the two generators 10, 12 we can visualize a more realistic AC power system. As the load 18 is increased (i.e., draws more current), there will be a phase shift between the load and the AC voltage sources. If the sources are perfect voltage sources (i.e., having no power limits or output impedance), this will work well and adjusting the phase relationship can control the relative power from each source.
When the two sources are a synchronous type generator such as one driven by an internal combustion engine, the system reacts differently. When the load draws more power, the frequency of each generator will drop until a control system for the engine fuel system increases the engine output to bring the system power back into balance. In this situation the power and frequency balances need to be controlled. Both sources are trying to provide the correct frequency and speed. There are many traditional solutions to this problem. If the machines are near each other or there is high-speed communications between the machines, one master controller can schedule and control the fuel to each machine. Another method, called isochronous control, couples the output power sense and control signal through a set of signals in such way to make the machines balance power. Both of these methods require high-speed communications or actual control wires between generators to control system power balance and frequency.
Another method that does not use communications and is used frequently is frequency droop control. In this method each generator in a system has a frequency power schedule so that when the frequency is low they make more power and when the frequency is high they make less power. This way the power outputs are balanced on a per unit basis for the connected generators if the frequency droop factor is the same for each machine in proportion to its size. The frequency range needed for this type of control depends on the accuracy with which the absolute frequency can be determined by each controller. The frequency can vary as much as +/−3 Hz when using this method with small rotating machines. These basic concepts are used to control power systems of many synchronous generators throughout utility power networks.
When controlling parallel inverters, one solution is to use the same droop control methods used in synchronous generators. This is achieved by controlling real power or real current of the inverter in proportion to the frequency error. This has the same effect as the synchronous generator control described above. A typical inverter control system is shown in FIG. 2. It should be noted that the frequency error signal from the phase locked loop 20 (PLL) goes to the power command signal of the inverter controller 22.
Typically a second order PLL is used for this application. The second order PLL 20 provides very little phase error at any frequency of operation at steady state. Sometimes a first order PLL is used but this has a phase error, which is a function of frequency. This phase error can be set to zero at any frequency by using the offset center frequency as shown in FIG. 3.
It is considered advantageous to provide a distributed power inverter which could be applied to loads with active power converter front ends. In this way, certain dispatchable loads could help improve system power and frequency control allowing better use of available generation capacity by reducing the need for reserve power (spinning reserve).
Also, it is considered advantageous to provide uninterruptible power supply (UPS) systems that consist of multiple devices operating together to provide the required systems functions, including operating in parallel both connected to and isolated from the utility grid to provide uninterrupted power to the connected loads.